1. Field of the Invention
The invention relates to an anti-dazzling transparent screen for elongate illuminant bodies which covers the illuminant body over the length thereof for the purpose of providing an anti-dazzling effect of a radiating sector of the illuminant body, wherein the surface of the transparent screen in formed by elongate prisms extending approximately parallel to one another and aligned substantially along the illuminant body.
2. Description of the Related Art
In the illumination of rooms, in particular, in the case of office illumination with the usually required high luminance, the light source is to be provided with anti-dazzling properties relative to the position of a work space such that no disturbance caused by glare occurs in the field of view of the worker when viewing the work assignment. In order to provide a comfortable room illumination, measures are required to change the luminance produced by the light source and observed by the worker. In particular, in office work spaces which are furnished with monitors, direct glare and reflective glare are to be prevented. Direct glare occurs when great brightness is generated in the field of sight, for example, when viewing the work surface, for example, the monitor or paperwork. In principle, a direct viewing of the illuminant body is to be prevented. In the case of elongate illuminant bodies anti-dazzling measures in the transverse direction, partially also in the longitudinal direction, of the illuminant body are known wherein the radiating angle of the illuminant body is realized by downwardly extended housing walls of a housing in which the illuminant body is received. Those work spaces which are outside of the radiating sector of the illuminant body are glare-free.
For providing an anti-dazzling effect of the illuminant body within the radiating sector, anti-dazzling transparent screens are known which are comprised of light-transmissive material and cover the illuminant body over the length thereof. From DE 34 20 414 C2 a light-transmissive lamp cover for providing an anti-glare effect of lighting devices with elongate lamps and a reflector arranged above the lamp is known which covers the reflector opening and is provided with elongate prisms, designed to scatter the light passing through, on the side facing away from the lamp. The elongate prisms are positioned approximately parallel to one another and extend transverse to the longitudinal axis of the lamp, wherein in this way, taking into account the refractive index of the material of the anti-dazzling transparent screen, the radiating angle of the illuminant body along the axis of the lamp is to be limited. The prism cross-section has the shape of an isosceles triangle, and the shape of the prism cross-section must be selected such that a total reflection is prevented in order to thus affect the light distribution of the lighting device transverse to the lamp axis as little as possible. In this way, the illuminant body is pictured on the visible surface of the anti-dazzling transparent screen wherein the very bright image of the illuminant body is often perceived as disturbing. In this connection, the observed luminance is between 80% to 100% of the luminance of the light source, measured in the field of sight of a viewer, in particular, when in a seated position. An anti-glare effect of the illuminant body in the transverse direction is not attempted with the known arrangement.
From DE 41 15 836 A1 a lighting device with a rod-shaped, horizontally arranged light source is known which, for the purpose of providing an anti-glare effect, is surrounded by a prism foil. The prisms are arranged parallel and adjacent to one another and extend parallel to the longitudinal axis of the housing. On the foil which is arranged concentrically to the rod-shaped light source, the prisms are formed as isosceles triangles and arranged symmetrically wherein a transparent protective tube is positioned about the cylindrical prism body. The light beams which are radially emitted by the rod-shaped lamp and the immediate neighboring beams penetrate into the respective prism approximately perpendicularly to the prism base and are reflected on the prism surfaces which are the legs of the rectangular prism cross-section. In this way, the radial beams which are the most intensive ones of all radiated light beams are reflected back into the light source and are absorbed therein so that an antilare effect can be achieved with this known arrangement only with an enormous loss of light.
DE 197 45 844 A1 discloses the use of prism foils for the light emission opening of a reflector. In this connection, the reflector and prism foil surround the illuminant body. The prism contour is essentially a planar surface having arranged at one side the ribs of the actual prism structure. The longitudinal axis of the prisms is perpendicular to the lamp axis. In order to obtain a wide radiating (inner light of a vehicle) or a directed (signal light of a vehicle) light distribution, the reflector is to be dimensioned such that the reflector and the prism foil form an integral unit.